A method for recycling waste lubricating oil

Abstract

The present invention discloses a method for regenerating waste lubricating oil, relates to the field of petrochemical technology, and is patented under the patent classification number C10M175 / 06. The preparation method comprises the following steps: adding a composite flocculant to the waste lubricating oil, heating and stirring, and then performing magnetic separation to obtain pretreated waste lubricating oil; uniformly mixing the pretreated waste lubricating oil, glycerin, and a Ni / Al2O3 catalyst, then heating and reacting under pressure, cooling to room temperature, and obtaining a catalytic hydrogenation product; subjecting the catalytic hydrogenation product to vacuum distillation to recover a lubricating oil base oil fraction, adding methanol and concentrated sulfuric acid to the distillation residue, heating for an esterification reaction, and separating the liquids to obtain biodiesel. The pretreatment process of the present invention uses a composite flocculant to remove acid and particulate impurities from the lubricating oil, replacing the traditional alkali neutralization and precipitation process, which is environmentally friendly and reduces the company's recycling costs.

Description

Technical Field

[0001] The present invention relates to the technical field of petrochemical industry, belongs to patent classification number C10M175 / 06, and specifically relates to a resource regeneration method for waste lubricating oil. Background Art

[0002] Lubricants play a vital role in today's industrial production and machinery operations, effectively reducing friction between mechanical components, minimizing wear and tear, and ensuring stable equipment operation. However, during use, lubricants gradually age and deteriorate due to factors such as metal shavings from mechanical wear, impurities from the environment, high temperatures, and air oxidation. When the deterioration reaches a certain level, the lubricant can no longer meet usage requirements and becomes waste lubricant. According to incomplete statistics, a considerable amount of waste lubricant is generated globally each year. If this waste lubricant is not properly handled, it not only results in a significant waste of resources but also causes serious environmental pollution, such as contamination of soil and water sources, disrupting the ecological balance.

[0003] Currently, catalytic hydrorefining is a common method for recycling waste lubricating oil. This method effectively removes impurities from waste lubricating oil, restoring some of its properties and enabling it to meet reusable standards. Alkali neutralization, precipitation, and filtration are commonly used in lubricating oil pretreatment. A strong base is added to neutralize the acidic substances in the waste lubricating oil. Sedimentation and filtration are then performed to remove solid impurities and moisture, providing relatively pure feedstock for subsequent hydrorefining.

[0004] However, this traditional alkali neutralization, precipitation, and filtration process has numerous drawbacks. The waste lye produced by the use of strong alkalis contains numerous harmful substances and, if improperly handled, can cause severe environmental pollution. Rendering this waste lye harmless often requires significant investment, including specialized treatment equipment, chemicals, and specialized processes. This undoubtedly increases the financial burden of the entire waste lubricant regeneration process and limits its widespread application and promotion. Summary of the Invention

[0005] The present invention aims to provide a method for recycling waste lubricating oil to address the technical issues raised in the aforementioned background art. The present invention utilizes a composite flocculant in the pretreatment process to remove acid and particulate impurities from the lubricating oil, replacing the traditional alkali neutralization and precipitation process. This method is environmentally friendly and reduces recycling costs for businesses.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for recycling waste lubricating oil comprises the following steps:

[0008] S1, adding a composite flocculant to waste lubricating oil, heating and stirring, and then performing magnetic separation to obtain pretreated waste lubricating oil;

[0009] S2, mixing the pretreated waste lubricating oil, glycerol and Ni / Al2O3 catalyst uniformly, then heating under pressure to react, and cooling to room temperature to obtain a catalytic hydrogenation product;

[0010] S3. The catalytic hydrogenation product is subjected to vacuum distillation to recover the lubricating oil base oil fraction, methanol and concentrated sulfuric acid are added to the distillation residue, heated to carry out esterification reaction, and liquid separation is performed to obtain biodiesel.

[0011] In the technical solution of the present invention, a composite flocculant is used in combination with magnetic separation technology to adsorb particulate impurities and remove acid from waste lubricating oil. Compared with the traditional alkali neutralization process, the magnetic separation process does not require the introduction of chemical waste liquid, reducing environmental pollution and wastewater treatment costs in the traditional pretreatment stage. A mixed system of glycerol and Ni / Al2O3 catalyst is used to carry out catalytic hydrogenation reaction under pressure heating conditions. Glycerol, as a green solvent, can increase the solubility of polar substances in waste oil and promote the uniformity of hydrogenation reaction. It not only effectively removes impurities such as sulfur and nitrogen, but also retains the effective components of the base oil and improves the quality of the regenerated oil. The lubricating oil base oil is separated by vacuum distillation, and the distillation residue is subjected to esterification reaction to prepare biodiesel, thereby realizing the joint recovery of base oil and biodiesel and significantly improving resource utilization. In addition, no waste alkali liquid is generated in the entire process chain, avoiding the environmental pollution problems of traditional processes.

[0012] Preferably, in step S1, the amount of the composite flocculant added is 0.5-3.0% of the mass of the waste lubricating oil.

[0013] Preferably, in step S2, the mass ratio of the pretreated waste lubricating oil, glycerol and Ni / Al2O3 catalyst is 10:1.5:0.3.

[0014] Preferably, in step S2, the heating reaction temperature is 220-230° C., and the heating reaction time is 3-4 h.

[0015] Preferably, in step S3, the vacuum distillation temperature is 280° C. and the pressure is 5 kPa.

[0016] Preferably, in step S3, the mass ratio of the distillation residue to methanol is 2:1.

[0017] Preferably, in step S1, the method for preparing the composite flocculant comprises the following steps:

[0018] S11, dissolving ferric chloride and ferric chloride in deionized water, heating to 70-80° C. under nitrogen protection, adding ammonia water, stirring and reacting, and performing magnetic separation and water washing to obtain Fe3O4 nanocrystals;

[0019] S12, dispersing Fe3O4 nanocrystals in a mixed solution of ethanol and water, adding ethyl orthosilicate and ammonia water, stirring and reacting at room temperature, and performing magnetic separation and drying to obtain Fe3O4@SiO2 composite particles;

[0020] S13, dispersing Fe3O4@SiO2 composite particles in a hexadecyltrimethylammonium bromide aqueous solution, then adding ethyl orthosilicate, heating and stirring to react, and then calcining after magnetic separation to remove the template agent to obtain mesoporous Fe3O4@SiO2 composite particles;

[0021] S14, adding silane coupling agent KH560 to a mixed solution of ethanol and water, heating and stirring to hydrolyze, then adding mesoporous Fe3O4@SiO2 composite particles, heating and stirring to react, to obtain epoxy-based mesoporous Fe3O4@SiO2 composite particles;

[0022] S15, dissolving chitosan in acetic acid solution, then adding glycidyl methacrylate, and heating for reaction to obtain double-bond modified chitosan;

[0023] S16, dissolving the double-bond modified chitosan in an acetic acid solution, then adding the epoxy-group mesoporous Fe3O4@SiO2 composite particles, heating and stirring to react, to obtain grafted double-bond modified chitosan mesoporous Fe3O4@SiO2 composite particles;

[0024] S17, dispersing the grafted double bond modified chitosan mesoporous Fe3O4@SiO2 composite particles in deionized water, then adding acrylamide and ammonium persulfate, heating and reacting under nitrogen protection to obtain a composite flocculant.

[0025] In the technical solution of the present invention, the preparation reaction principle of the composite flocculant is as follows: first, ferric chloride and ferric dichloride react in ammonia water under nitrogen protection to form Fe3O4 nanocrystals. Then, SiO2 is coated on the surface of the Fe3O4 nanocrystals to form a core-shell structure. A secondary SiO2 deposition and calcination are guided by a hexadecyltrimethylammonium bromide template to form a mesoporous layer. Then, epoxy groups are grafted onto the mesoporous SiO2 using the silane coupling agent KH560. This then undergoes an amine-epoxy ring-opening reaction with chitosan modified with glycidyl methacrylate. Finally, free radical copolymerization with acrylamide is initiated by ammonium persulfate to form a composite structure of Fe3O4@mesoporous SiO2-grafted chitosan-polyacrylamide. When used for pretreatment, the mesoporous SiO2 provides selective adsorption sites for small molecule acids by matching its mesoporous size with the chitosan amino group. Simultaneously, chemical neutralization of the chitosan amino group synergistically removes organic acids. In addition, the electrical neutralization and bridging flocculation effect of the polyacrylamide chain segments (long-chain adsorption particles form flocs) are used to remove solid impurities, and the Fe3O4 nanocrystalline core provides a magnetic response, which can achieve rapid separation under the action of the electric field, thereby efficiently purifying the waste lubricating oil.

[0026] Preferably, in step S11, the reaction time is 30-50 min.

[0027] Preferably, in step S13, the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide is 10:1-3.

[0028] In the technical solution of the present invention, as described above, a hexadecyltrimethylammonium bromide template is used to guide the deposition of secondary SiO2 and calcination to form a mesoporous layer. The pore structure of the mesoporous layer provides selective adsorption sites for small molecule acids, which synergistically remove acidic substances in waste lubricating oil. To achieve a good removal rate of acidic substances, a sufficient amount of hexadecyltrimethylammonium bromide template must be added. Therefore, the present invention controls the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide to be less than 10 / 1. As the amount of hexadecyltrimethylammonium bromide continues to increase, the present invention team unexpectedly discovered that when the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide is less than 10 / 3, the flocculation effect of the prepared composite flocculant on solid particles suddenly decreases significantly. After research, it was found that this is because the excess hexadecyltrimethylammonium bromide occupies its surface reaction sites after carbonization, thereby reducing the subsequent grafting amount of polyacrylamide, which in turn leads to a reduction in the flocculation effect on solid particles. Therefore, in order to comprehensively balance the acid removal effect and the solid particle flocculation effect, the present invention strictly controls the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide within the range of 10:1-3.

[0029] Preferably, in step S16, the mass ratio of epoxy-based mesoporous Fe3O4@SiO2 composite particles to double-bond modified chitosan is 7:2-4.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The catalyst hydrorefining method is used to recover lubricating oil in the pretreatment stage. Composite flocculants are combined with magnetic separation technology, eliminating the need for the traditional alkali neutralization process to introduce chemical waste liquid, significantly reducing environmental pollution and wastewater treatment costs in the pretreatment stage.

[0032] 2. Lubricant base oil is separated by vacuum distillation, and the distillation residue is esterified to produce biodiesel, thus achieving the joint recovery of base oil and biodiesel and significantly improving resource utilization.

[0033] 3. The composite flocculant can simultaneously remove acidic substances and solid particles in waste lubricating oil. The Fe3O4 nanocrystalline core provides magnetic response and can achieve rapid separation under the action of the electric field, thereby efficiently purifying the waste lubricating oil.

[0034] 4. Control the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide within the specified range to comprehensively balance the acid removal effect and the flocculation effect of solid particles. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the implementation regulations described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0036] Example 1

[0037] The preparation method of the composite flocculant comprises the following steps:

[0038] Step S11: Take a three-necked flask and add 16.2g of ferric chloride (FeCl3 6H2O, analytical grade, purity ≥99%) and 6.0g ferric chloride (FeCl2 4H2O, analytical grade, purity ≥99%), pour in 100mL of deionized water, stir magnetically until completely dissolved. Pass high-purity nitrogen (purity ≥99.99%) for protection, heat the oil bath to 75℃, stir at 300rpm, and slowly add 20mL of ammonia water (NH3 The reaction mixture was stirred for 45 minutes, maintaining the pH of the reaction system at ≥10 using 25% H₂O (analytical grade). After the reaction, the mixture was transferred to a magnetic separation device and separated at a magnetic field strength of 1.2 T for 10 minutes. The black precipitate was collected and washed three times with deionized water until the pH of the washing solution was close to neutral, yielding Fe₃O₄ nanocrystals.

[0039] Step S12: Disperse 5g of the aforementioned Fe3O4 nanocrystals in a mixture of 150mL of ethanol (analytical grade, 95%) and 50mL of deionized water. Add 2mL of 25% ammonia aqueous solution and stir at 200rpm at room temperature for 30 minutes. Slowly add 5mL of tetraethyl orthosilicate (TEOS, analytical grade, purity ≥98%) dropwise, and continue stirring for 24 hours. After the reaction is complete, separate the mixture in a 1.2T magnetic field for 10 minutes. Collect the solid product and dry it in a vacuum drying oven at 60°C for 12 hours to obtain Fe3O4@SiO2 composite particles.

[0040] Step S13: Dissolve 0.013 mol of hexadecyltrimethylammonium bromide (CTAB, analytical grade, purity ≥99%) in 500 mL of deionized water, add 5 g of Fe3O4@SiO2 composite particles, heat in a 60°C water bath, and stir at 300 rpm for 30 minutes. Add 0.05 mol of ethyl orthosilicate dropwise, raise the temperature to 80°C, and continue stirring for 12 hours. After separating the reaction solution in a 1.2 T magnetic field for 10 minutes, the solid product is transferred to a muffle furnace and heated to 550°C at a rate of 1°C / min. Calcinate for 6 hours to remove the CTAB template, yielding mesoporous Fe3O4@SiO2 composite particles.

[0041] Step S14: In a round-bottom flask, add 2 mL of silane coupling agent KH560 (analytical grade, purity ≥98%), 50 mL of ethanol, and 10 mL of deionized water. Heat in an oil bath at 70°C with a stirring rate of 200 rpm and allow the hydrolysis reaction to proceed for 2 hours. Add 5 g of mesoporous Fe₃O₄@SiO₂ composite particles, raise the temperature to 80°C, and reflux for 6 hours. After the reaction, collect the solid using magnetic separation (1.2 T, 10 minutes) and dry at 60°C overnight to obtain epoxy-based mesoporous Fe₃O₄@SiO₂ composite particles.

[0042] Step S15: Dissolve 2 g of chitosan (degree of deacetylation ≥90%, analytical grade) in 50 mL of 1% acetic acid solution (v / v). Heat in a 40°C water bath and stir until completely dissolved to form a transparent solution. Add 1.5 mL of glycidyl methacrylate (GMA, analytical grade, purity ≥98%), heat to 60°C, and reflux for 8 h. The reaction solution is dialyzed for 48 h using a dialysis bag to remove unreacted monomers, and then freeze-dried for 48 h to obtain a double-bond-modified chitosan solid.

[0043] Step S16: Dissolve 3.5 g of double-bond-modified chitosan in 100 mL of 1% acetic acid solution, add 7 g of epoxy-group mesoporous Fe₃O₄@SiO₂ composite particles, heat in an 80°C oil bath, and stir at 300 rpm for 12 h. After the reaction is complete, separate the mixture in a 1.2 T magnetic field for 10 min, collect the solid product, and vacuum dry it at 60°C for 12 h to obtain grafted double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles.

[0044] Step S17: Disperse 5 g of double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles in 50 mL of deionized water. Add 2.5 g of acrylamide (AM, analytical grade, purity ≥98%) and 0.1 g of ammonium persulfate (APS, analytical grade, purity ≥98%). Aerate the mixture under nitrogen for 30 minutes. Heat in a 60°C oil bath and stir at 200 rpm for 4 hours to form a black suspension, which is the composite flocculant.

[0045] The method for recycling waste lubricating oil comprises the following steps:

[0046] S1: Place 100g of waste lubricating oil (acid value 1.55mgKOH / g, 0.896% particulate impurities) in a beaker, heat to 50°C in an oil bath, add 2.5g of a composite flocculant, and stir at 300 rpm for 20 minutes. After the reaction is complete, transfer the mixture to a magnetic separation device and separate it at a magnetic field strength of 1.2T for 15 minutes. The supernatant is the pretreated waste lubricating oil.

[0047] S2: 100 g of pretreated waste lubricating oil, 15 g of glycerol (analytical grade, purity ≥99.5%), and 3 g of Ni / Al2O3 catalyst (Ni loading 15%) were added to a 500 mL autoclave. The atmosphere was purged with nitrogen three times (0.5 MPa pressure, 5 min each time). Heating and stirring were initiated, and the temperature was raised to 225°C. The reaction pressure was maintained at 3 MPa (hydrogen source provided by glycerol decomposition). The stirring rate was 500 rpm, and the reaction was continued for 3.5 h. After the reaction, the reaction mixture was cooled to room temperature to obtain the catalytic hydrogenation product.

[0048] S3: Base Oil Recovery by Vacuum Distillation: The catalytic hydrogenation product was transferred to a 500mL vacuum distillation apparatus equipped with a 30cm packed column. The distillation temperature was controlled at 280°C and the system pressure was 5kPa. The 180-320°C fraction was collected to obtain the lubricating base oil, and the distillation residue was retained for the esterification reaction.

[0049] To a round-bottom flask, add 15g of the distillation residue, 7.5g of methanol (analytical grade, purity ≥99.5%), and 0.15g of concentrated sulfuric acid (analytical grade, 98%, 1% of the residue mass). Heat in a 65°C oil bath and stir at 300 rpm for 2 hours. After cooling, transfer the reaction mixture to a separatory funnel and allow the layers to stand. Separate the upper layer of biodiesel and wash with deionized water until neutral to obtain biodiesel.

[0050] Example 2

[0051] The preparation method of the composite flocculant comprises the following steps:

[0052] Step S11: Take a three-necked flask and add 16.2g of ferric chloride (FeCl3 6H2O, analytical grade, purity ≥99%) and 6.0g ferric chloride (FeCl2 4H2O, analytical grade, purity ≥99%), pour in 100mL of deionized water, stir magnetically until completely dissolved. Pass high-purity nitrogen (purity ≥99.99%) for protection, heat the oil bath to 75℃, stir at 300rpm, and slowly add 20mL of ammonia water (NH3 25% H₂O (analytical grade), maintaining the reaction system pH ≥ 10, and stirring continuously for 40 minutes. After the reaction, the mixture was transferred to a magnetic separation device and separated at a magnetic field strength of 1.2 T for 10 minutes. The black precipitate was collected and washed three times with deionized water until the pH of the washing solution was close to neutral, yielding Fe₃O₄ nanocrystals.

[0053] Step S12: Disperse 5g of the aforementioned Fe3O4 nanocrystals in a mixture of 150mL of ethanol (analytical grade, 95%) and 50mL of deionized water. Add 2mL of 25% ammonia aqueous solution and stir at 200rpm at room temperature for 30 minutes. Slowly add 5mL of tetraethyl orthosilicate (TEOS, analytical grade, purity ≥98%) dropwise, and continue stirring for 24 hours. After the reaction is complete, separate the mixture in a 1.2T magnetic field for 10 minutes. Collect the solid product and dry it in a vacuum drying oven at 60°C for 12 hours to obtain Fe3O4@SiO2 composite particles.

[0054] Step S13: Dissolve 0.008 mol of hexadecyltrimethylammonium bromide (CTAB, analytical grade, purity ≥99%) in 500 mL of deionized water, add 5 g of Fe3O4@SiO2 composite particles, heat in a 60°C water bath, and stir at 300 rpm for 30 minutes. Add 0.05 mol of ethyl orthosilicate dropwise, raise the temperature to 80°C, and continue stirring for 12 hours. After separating the reaction solution in a 1.2 T magnetic field for 10 minutes, the solid product is transferred to a muffle furnace and heated to 550°C at a rate of 1°C / min. Calcinate for 6 hours to remove the CTAB template, yielding mesoporous Fe3O4@SiO2 composite particles.

[0055] Step S14: In a round-bottom flask, add 2 mL of silane coupling agent KH560 (analytical grade, purity ≥98%), 50 mL of ethanol, and 10 mL of deionized water. Heat in an oil bath at 70°C with a stirring rate of 200 rpm and allow the hydrolysis reaction to proceed for 2 hours. Add 5 g of mesoporous Fe₃O₄@SiO₂ composite particles, raise the temperature to 80°C, and reflux for 6 hours. After the reaction, collect the solid using magnetic separation (1.2 T, 10 minutes) and dry at 60°C overnight to obtain epoxy-based mesoporous Fe₃O₄@SiO₂ composite particles.

[0056] Step S15: Dissolve 2 g of chitosan (degree of deacetylation ≥90%, analytical grade) in 50 mL of 1% acetic acid solution (v / v). Heat in a 40°C water bath and stir until completely dissolved to form a transparent solution. Add 1.5 mL of glycidyl methacrylate (GMA, analytical grade, purity ≥98%), heat to 60°C, and reflux for 8 h. The reaction solution is dialyzed for 48 h using a dialysis bag to remove unreacted monomers, and then freeze-dried for 48 h to obtain a double-bond-modified chitosan solid.

[0057] Step S16: Dissolve 2.5 g of double-bond-modified chitosan in 100 mL of 1% acetic acid solution, add 7 g of epoxy-group mesoporous Fe₃O₄@SiO₂ composite particles, heat in an 80°C oil bath, and stir at 300 rpm for 12 h. After the reaction is complete, separate the mixture in a 1.2 T magnetic field for 10 min, collect the solid product, and vacuum dry it at 60°C for 12 h to obtain grafted double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles.

[0058] Step S17: Disperse 5 g of double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles in 50 mL of deionized water. Add 2.5 g of acrylamide (AM, analytical grade, purity ≥98%) and 0.1 g of ammonium persulfate (APS, analytical grade, purity ≥98%). Aerate the mixture under nitrogen for 30 minutes. Heat in a 60°C oil bath and stir at 200 rpm for 4 hours to form a black suspension, which is the composite flocculant.

[0059] The method for recycling waste lubricating oil comprises the following steps:

[0060] S1: Place 100g of waste lubricating oil (acid value 1.55mgKOH / g, 0.896% particulate impurities) in a beaker, heat to 50°C in an oil bath, add 1.0g of a composite flocculant, and stir at 300 rpm for 20 minutes. After the reaction is complete, transfer the mixture to a magnetic separation device and separate it at a magnetic field strength of 1.2T for 15 minutes. The supernatant is the pretreated waste lubricating oil.

[0061] S2: 100 g of pretreated waste lubricating oil, 15 g of glycerol (analytical grade, purity ≥99.5%), and 3 g of Ni / Al2O3 catalyst (Ni loading 15%) were added to a 500 mL autoclave. The atmosphere was purged with nitrogen three times (0.5 MPa pressure, 5 min each time). Heating and stirring were initiated, and the temperature was raised to 225°C. The reaction pressure was maintained at 3 MPa (hydrogen source provided by glycerol decomposition). The stirring rate was 500 rpm, and the reaction was continued for 3.5 h. After the reaction, the reaction mixture was cooled to room temperature to obtain the catalytic hydrogenation product.

[0062] S3: Base Oil Recovery by Vacuum Distillation: The catalytic hydrogenation product was transferred to a 500mL vacuum distillation apparatus equipped with a 30cm packed column. The distillation temperature was controlled at 280°C and the system pressure was 5kPa. The 180-320°C fraction was collected to obtain the lubricating base oil, and the distillation residue was retained for the esterification reaction.

[0063] To a round-bottom flask, add 15g of the distillation residue, 7.5g of methanol (analytical grade, purity ≥99.5%), and 0.15g of concentrated sulfuric acid (analytical grade, 98%, 1% of the residue mass). Heat in a 65°C oil bath and stir at 300 rpm for 2 hours. After cooling, transfer the reaction mixture to a separatory funnel and allow the layers to stand. Separate the upper layer of biodiesel and wash with deionized water until neutral to obtain biodiesel.

[0064] Example 3

[0065] The preparation method of the composite flocculant comprises the following steps:

[0066] Step S11: Take a three-necked flask and add 16.2g of ferric chloride (FeCl3 6H2O, analytical grade, purity ≥99%) and 6.0g ferric chloride (FeCl2 4H2O, analytical grade, purity ≥99%), pour in 100mL of deionized water, stir magnetically until completely dissolved. Pass high-purity nitrogen (purity ≥99.99%) for protection, heat the oil bath to 75℃, stir at 300rpm, and slowly add 20mL of ammonia water (NH3 25% H₂O (analytical grade), maintaining the reaction system pH ≥ 10, and stirring continuously for 40 minutes. After the reaction, the mixture was transferred to a magnetic separation device and separated at a magnetic field strength of 1.2 T for 10 minutes. The black precipitate was collected and washed three times with deionized water until the pH of the washing solution was close to neutral, yielding Fe₃O₄ nanocrystals.

[0067] Step S12: Disperse 5g of the aforementioned Fe3O4 nanocrystals in a mixture of 150mL of ethanol (analytical grade, 95%) and 50mL of deionized water. Add 2mL of 25% ammonia aqueous solution and stir at 200rpm at room temperature for 30 minutes. Slowly add 5mL of tetraethyl orthosilicate (TEOS, analytical grade, purity ≥98%) dropwise, and continue stirring for 24 hours. After the reaction is complete, separate the mixture in a 1.2T magnetic field for 10 minutes. Collect the solid product and dry it in a vacuum drying oven at 60°C for 12 hours to obtain Fe3O4@SiO2 composite particles.

[0068] Step S13: Dissolve 0.01 mol of hexadecyltrimethylammonium bromide (CTAB, analytical grade, purity ≥99%) in 500 mL of deionized water, add 5 g of Fe3O4@SiO2 composite particles, heat in a 60°C water bath, and stir at 300 rpm for 30 minutes. Add 0.05 mol of ethyl orthosilicate dropwise, raise the temperature to 80°C, and continue stirring for 12 hours. After separating the reaction solution in a 1.2 T magnetic field for 10 minutes, the solid product is transferred to a muffle furnace and heated to 550°C at a rate of 1°C / min. Calcinate for 6 hours to remove the CTAB template, yielding mesoporous Fe3O4@SiO2 composite particles.

[0069] Step S14: In a round-bottom flask, add 2 mL of silane coupling agent KH560 (analytical grade, purity ≥98%), 50 mL of ethanol, and 10 mL of deionized water. Heat in an oil bath at 70°C with a stirring rate of 200 rpm and allow the hydrolysis reaction to proceed for 2 hours. Add 5 g of mesoporous Fe₃O₄@SiO₂ composite particles, raise the temperature to 80°C, and reflux for 6 hours. After the reaction, collect the solid using magnetic separation (1.2 T, 10 minutes) and dry at 60°C overnight to obtain epoxy-based mesoporous Fe₃O₄@SiO₂ composite particles.

[0070] Step S15: Dissolve 2 g of chitosan (degree of deacetylation ≥90%, analytical grade) in 50 mL of 1% acetic acid solution (v / v). Heat in a 40°C water bath and stir until completely dissolved to form a transparent solution. Add 1.5 mL of glycidyl methacrylate (GMA, analytical grade, purity ≥98%), heat to 60°C, and reflux for 8 h. The reaction solution is dialyzed for 48 h using a dialysis bag to remove unreacted monomers, and then freeze-dried for 48 h to obtain a double-bond-modified chitosan solid.

[0071] Step S16: Dissolve 3 g of double-bond-modified chitosan in 100 mL of 1% acetic acid solution, add 7 g of epoxy-group mesoporous Fe₃O₄@SiO₂ composite particles, heat in an 80°C oil bath, and stir at 300 rpm for 12 h. After the reaction is complete, separate the mixture in a 1.2 T magnetic field for 10 min, collect the solid product, and vacuum dry it at 60°C for 12 h to obtain grafted double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles.

[0072] Step S17: Disperse 5 g of double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles in 50 mL of deionized water. Add 2.5 g of acrylamide (AM, analytical grade, purity ≥98%) and 0.1 g of ammonium persulfate (APS, analytical grade, purity ≥98%). Aerate the mixture under nitrogen for 30 minutes. Heat in a 60°C oil bath and stir at 200 rpm for 4 hours to form a black suspension, which is the composite flocculant.

[0073] The method for recycling waste lubricating oil comprises the following steps:

[0074] S1: Place 100g of waste lubricating oil (acid value 1.55mgKOH / g, 0.896% particulate impurities) in a beaker and heat to 50°C in an oil bath. Add 2g of composite flocculant and stir at 300 rpm for 20 minutes. After the reaction is complete, transfer the mixture to a magnetic separation device and separate it at a magnetic field strength of 1.2T for 15 minutes. The supernatant is the pretreated waste lubricating oil.

[0075] S2: 100 g of pretreated waste lubricating oil, 15 g of glycerol (analytical grade, purity ≥99.5%), and 3 g of Ni / Al2O3 catalyst (Ni loading 15%) were added to a 500 mL autoclave. The atmosphere was purged with nitrogen three times (0.5 MPa pressure, 5 min each time). Heating and stirring were initiated, and the temperature was raised to 225°C. The reaction pressure was maintained at 3 MPa (hydrogen source provided by glycerol decomposition). The stirring rate was 500 rpm, and the reaction was continued for 3.5 h. After the reaction, the reaction mixture was cooled to room temperature to obtain the catalytic hydrogenation product.

[0076] S3: Base Oil Recovery by Vacuum Distillation: The catalytic hydrogenation product was transferred to a 500mL vacuum distillation apparatus equipped with a 30cm packed column. The distillation temperature was controlled at 280°C and the system pressure was 5kPa. The 180-320°C fraction was collected to obtain the lubricating base oil, and the distillation residue was retained for the esterification reaction.

[0077] To a round-bottom flask, add 15g of the distillation residue, 7.5g of methanol (analytical grade, purity ≥99.5%), and 0.15g of concentrated sulfuric acid (analytical grade, 98%, 1% of the residue mass). Heat in a 65°C oil bath and stir at 300 rpm for 2 hours. After cooling, transfer the reaction mixture to a separatory funnel and allow the layers to stand. Separate the upper layer of biodiesel and wash with deionized water until neutral to obtain biodiesel.

[0078] Example 4

[0079] The preparation method of the composite flocculant comprises the following steps:

[0080] Step S11: Take a three-necked flask and add 16.2g of ferric chloride (FeCl3 6H2O, analytical grade, purity ≥99%) and 6.0g ferric chloride (FeCl2 4H2O, analytical grade, purity ≥99%), pour in 100mL of deionized water, stir magnetically until completely dissolved. Pass high-purity nitrogen (purity ≥99.99%) for protection, heat the oil bath to 80℃, stir at 300rpm, and slowly add 20mL of ammonia water (NH3 25% H₂O (analytical grade), maintaining the reaction system pH ≥ 10, and stirring continuously for 50 minutes. After the reaction, the mixture was transferred to a magnetic separation device and separated at a magnetic field strength of 1.2 T for 10 minutes. The black precipitate was collected and washed three times with deionized water until the pH of the washing solution was close to neutral, yielding Fe₃O₄ nanocrystals.

[0081] Step S12: Disperse 5g of the aforementioned Fe3O4 nanocrystals in a mixture of 150mL of ethanol (analytical grade, 95%) and 50mL of deionized water. Add 2mL of 25% ammonia aqueous solution and stir at 200rpm at room temperature for 30 minutes. Slowly add 5mL of tetraethyl orthosilicate (TEOS, analytical grade, purity ≥98%) dropwise, and continue stirring for 24 hours. After the reaction is complete, separate the mixture in a 1.2T magnetic field for 10 minutes. Collect the solid product and dry it in a vacuum drying oven at 60°C for 12 hours to obtain Fe3O4@SiO2 composite particles.

[0082] Step S13: Dissolve 0.015 mol of hexadecyltrimethylammonium bromide (CTAB, analytical grade, purity ≥99%) in 500 mL of deionized water, add 5 g of Fe3O4@SiO2 composite particles, heat in a 60°C water bath, and stir at 300 rpm for 30 minutes. Add 0.05 mol of ethyl orthosilicate dropwise, raise the temperature to 80°C, and continue stirring for 12 hours. After separating the reaction solution in a 1.2 T magnetic field for 10 minutes, the solid product is transferred to a muffle furnace and heated to 550°C at a rate of 1°C / min. Calcinate for 6 hours to remove the CTAB template, yielding mesoporous Fe3O4@SiO2 composite particles.

[0083] Step S14: In a round-bottom flask, add 2 mL of silane coupling agent KH560 (analytical grade, purity ≥98%), 50 mL of ethanol, and 10 mL of deionized water. Heat in an oil bath at 70°C with a stirring rate of 200 rpm and allow the hydrolysis reaction to proceed for 2 hours. Add 5 g of mesoporous Fe₃O₄@SiO₂ composite particles, raise the temperature to 80°C, and reflux for 6 hours. After the reaction, collect the solid using magnetic separation (1.2 T, 10 minutes) and dry at 60°C overnight to obtain epoxy-based mesoporous Fe₃O₄@SiO₂ composite particles.

[0084] Step S15: Dissolve 2 g of chitosan (degree of deacetylation ≥90%, analytical grade) in 50 mL of 1% acetic acid solution (v / v). Heat in a 40°C water bath and stir until completely dissolved to form a transparent solution. Add 1.5 mL of glycidyl methacrylate (GMA, analytical grade, purity ≥98%), heat to 60°C, and reflux for 8 h. The reaction solution is dialyzed for 48 h using a dialysis bag to remove unreacted monomers, and then freeze-dried for 48 h to obtain a double-bond-modified chitosan solid.

[0085] Step S16: Dissolve 4 g of double-bond-modified chitosan in 100 mL of 1% acetic acid solution, add 7 g of epoxy-group mesoporous Fe₃O₄@SiO₂ composite particles, heat in an 80°C oil bath, and stir at 300 rpm for 12 h. After the reaction is complete, separate the mixture in a 1.2 T magnetic field for 10 min, collect the solid product, and vacuum dry it at 60°C for 12 h to obtain grafted double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles.

[0086] Step S17: Disperse 5 g of double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles in 50 mL of deionized water. Add 2.5 g of acrylamide (AM, analytical grade, purity ≥98%) and 0.1 g of ammonium persulfate (APS, analytical grade, purity ≥98%). Aerate the mixture under nitrogen for 30 minutes. Heat in a 60°C oil bath and stir at 200 rpm for 4 hours to form a black suspension, which is the composite flocculant.

[0087] The method for recycling waste lubricating oil comprises the following steps:

[0088] S1: Place 100g of waste lubricating oil (acid value 1.55mgKOH / g, 0.896% particulate impurities) in a beaker, heat to 50°C in an oil bath, add 3g of composite flocculant, and stir at 300 rpm for 20 minutes. After the reaction is complete, transfer the mixture to a magnetic separation device and separate it at a magnetic field strength of 1.2T for 15 minutes. The supernatant is the pretreated waste lubricating oil.

[0089] S2: 100 g of pretreated waste lubricating oil, 15 g of glycerol (analytical grade, purity ≥99.5%), and 3 g of Ni / Al2O3 catalyst (Ni loading 15%) were added to a 500 mL autoclave. The atmosphere was purged with nitrogen three times (0.5 MPa pressure, 5 min each time). Heating and stirring were initiated, and the temperature was raised to 230°C. The reaction pressure was maintained at 3 MPa (hydrogen source provided by glycerol decomposition). The stirring rate was 500 rpm, and the reaction was continued for 4 h. After the reaction, the reaction mixture was cooled to room temperature to obtain the catalytic hydrogenation product.

[0090] S3: Base Oil Recovery by Vacuum Distillation: The catalytic hydrogenation product was transferred to a 500mL vacuum distillation apparatus equipped with a 30cm packed column. The distillation temperature was controlled at 280°C and the system pressure was 5kPa. The 180-320°C fraction was collected to obtain the lubricating base oil, and the distillation residue was retained for the esterification reaction.

[0091] To a round-bottom flask, add 15g of the distillation residue, 7.5g of methanol (analytical grade, purity ≥99.5%), and 0.15g of concentrated sulfuric acid (analytical grade, 98%, 1% of the residue mass). Heat in a 65°C oil bath and stir at 300 rpm for 2 hours. After cooling, transfer the reaction mixture to a separatory funnel and allow the layers to stand. Separate the upper layer of biodiesel and wash with deionized water until neutral to obtain biodiesel.

[0092] Example 5

[0093] The preparation method of the composite flocculant comprises the following steps:

[0094] Step S11: Take a three-necked flask and add 16.2g of ferric chloride (FeCl3 6H2O, analytical grade, purity ≥99%) and 6.0g ferric chloride (FeCl2 4H2O, analytical grade, purity ≥99%), pour in 100mL of deionized water, stir magnetically until completely dissolved. Pass high-purity nitrogen (purity ≥99.99%) for protection, heat the oil bath to 70℃, stir at 300rpm, and slowly add 20mL of ammonia water (NH3 25% H₂O (analytical grade), maintaining the reaction system pH ≥ 10, and stirring continuously for 30 minutes. After the reaction, the mixture was transferred to a magnetic separation device and separated at a magnetic field strength of 1.2 T for 10 minutes. The black precipitate was collected and washed three times with deionized water until the pH of the washing solution was close to neutral, yielding Fe₃O₄ nanocrystals.

[0095] Step S12: Disperse 5g of the aforementioned Fe3O4 nanocrystals in a mixture of 150mL of ethanol (analytical grade, 95%) and 50mL of deionized water. Add 2mL of 25% ammonia aqueous solution and stir at 200rpm at room temperature for 30 minutes. Slowly add 5mL of tetraethyl orthosilicate (TEOS, analytical grade, purity ≥98%) dropwise, and continue stirring for 24 hours. After the reaction is complete, separate the mixture in a 1.2T magnetic field for 10 minutes. Collect the solid product and dry it in a vacuum drying oven at 60°C for 12 hours to obtain Fe3O4@SiO2 composite particles.

[0096] Step S13: Dissolve 0.005 mol of hexadecyltrimethylammonium bromide (CTAB, analytical grade, purity ≥99%) in 500 mL of deionized water, add 5 g of Fe3O4@SiO2 composite particles, heat in a 60°C water bath, and stir at 300 rpm for 30 minutes. Add 0.05 mol of ethyl orthosilicate dropwise, raise the temperature to 80°C, and continue stirring for 12 hours. After separating the reaction solution in a 1.2 T magnetic field for 10 minutes, the solid product is transferred to a muffle furnace and heated to 550°C at a rate of 1°C / min. Calcinate for 6 hours to remove the CTAB template, yielding mesoporous Fe3O4@SiO2 composite particles.

[0097] Step S14: In a round-bottom flask, add 2 mL of silane coupling agent KH560 (analytical grade, purity ≥98%), 50 mL of ethanol, and 10 mL of deionized water. Heat in an oil bath at 70°C with a stirring rate of 200 rpm and allow the hydrolysis reaction to proceed for 2 hours. Add 5 g of mesoporous Fe₃O₄@SiO₂ composite particles, raise the temperature to 80°C, and reflux for 6 hours. After the reaction, collect the solid using magnetic separation (1.2 T, 10 minutes) and dry at 60°C overnight to obtain epoxy-based mesoporous Fe₃O₄@SiO₂ composite particles.

[0098] Step S15: Dissolve 2 g of chitosan (degree of deacetylation ≥90%, analytical grade) in 50 mL of 1% acetic acid solution (v / v). Heat in a 40°C water bath and stir until completely dissolved to form a transparent solution. Add 1.5 mL of glycidyl methacrylate (GMA, analytical grade, purity ≥98%), heat to 60°C, and reflux for 8 h. The reaction solution is dialyzed for 48 h using a dialysis bag to remove unreacted monomers, and then freeze-dried for 48 h to obtain a double-bond-modified chitosan solid.

[0099] Step S16: Dissolve 2 g of double-bond-modified chitosan in 100 mL of 1% acetic acid solution, add 7 g of epoxy-group mesoporous Fe₃O₄@SiO₂ composite particles, heat in an 80°C oil bath, and stir at 300 rpm for 12 h. After the reaction is complete, separate the mixture in a 1.2 T magnetic field for 10 min, collect the solid product, and vacuum dry it at 60°C for 12 h to obtain grafted double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles.

[0100] Step S17: Disperse 5 g of double-bond-modified chitosan mesoporous Fe₃O₄@SiO₂ composite particles in 50 mL of deionized water. Add 2.5 g of acrylamide (AM, analytical grade, purity ≥98%) and 0.1 g of ammonium persulfate (APS, analytical grade, purity ≥98%). Aerate the mixture under nitrogen for 30 minutes. Heat in a 60°C oil bath and stir at 200 rpm for 4 hours to form a black suspension, which is the composite flocculant.

[0101] The method for recycling waste lubricating oil comprises the following steps:

[0102] S1: Place 100g of waste lubricating oil (acid value 1.55mgKOH / g, 0.896% particulate impurities) in a beaker, heat to 50°C in an oil bath, add 0.5g of a composite flocculant, and stir at 300 rpm for 20 minutes. After the reaction is complete, transfer the mixture to a magnetic separation device and separate it at a magnetic field strength of 1.2T for 15 minutes. The supernatant is the pretreated waste lubricating oil.

[0103] S2: 100 g of pretreated waste lubricating oil, 15 g of glycerol (analytical grade, purity ≥99.5%), and 3 g of Ni / Al2O3 catalyst (Ni loading 15%) were added to a 500 mL autoclave. The atmosphere was purged with nitrogen three times (0.5 MPa pressure, 5 min each time). Heating and stirring were initiated, and the temperature was raised to 220°C. The reaction pressure was maintained at 3 MPa (hydrogen source provided by glycerol decomposition). The stirring rate was 500 rpm, and the reaction was continued for 3 h. After the reaction, the reaction was allowed to cool naturally to room temperature to obtain the catalytic hydrogenation product.

[0104] S3: Base Oil Recovery by Vacuum Distillation: The catalytic hydrogenation product was transferred to a 500mL vacuum distillation apparatus equipped with a 30cm packed column. The distillation temperature was controlled at 280°C and the system pressure was 5kPa. The 180-320°C fraction was collected to obtain the lubricating base oil, and the distillation residue was retained for the esterification reaction.

[0105] To a round-bottom flask, add 15g of the distillation residue, 7.5g of methanol (analytical grade, purity ≥99.5%), and 0.15g of concentrated sulfuric acid (analytical grade, 98%, 1% of the residue mass). Heat in a 65°C oil bath and stir at 300 rpm for 2 hours. After cooling, transfer the reaction mixture to a separatory funnel and allow the layers to stand. Separate the upper layer of biodiesel and wash with deionized water until neutral to obtain biodiesel.

[0106] Comparative Example 1

[0107] The difference between Comparative Example 1 and Example 1 is that in the resource regeneration process of waste lubricating oil, the composite flocculant is replaced by Fe3O4@SiO2 composite particles, and the other steps are the same.

[0108] Comparative Example 2

[0109] The difference between Comparative Example 2 and Example 1 is that in the resource regeneration process of waste lubricating oil, the composite flocculant is replaced by mesoporous Fe3O4@SiO2 composite particles, and the other steps are the same.

[0110] Comparative Example 3

[0111] The difference between Comparative Example 3 and Example 1 is that in the resource regeneration process of waste lubricating oil, the composite flocculant is replaced by grafted double-bond modified chitosan mesoporous Fe3O4@SiO2 composite particles, and the other steps are the same.

[0112] Comparative Example 4

[0113] The difference between Comparative Example 4 and Example 4 is that the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide is 10:4.

[0114] Comparative Example 5

[0115] The difference between Comparative Example 5 and Example 4 is that the molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide is 10:5.

[0116] Performance testing:

[0117] 1.1 Lubricant base oil and biodiesel yield test:

[0118] The pretreated waste lubricating oil was weighed, and the fractions were collected by vacuum distillation. After cooling, the base oil was weighed. The lubricating base oil yield was calculated using the formula: recovered base oil mass / pretreated waste lubricating oil mass × 100%. The biodiesel yield was calculated by weighing the distillation residue after vacuum distillation using the formula: biodiesel mass / distillation residue mass × 100%. The test results are shown in Table 1.

[0119] 1.2 Lubricant base oil and biodiesel acid value test:

[0120] Using the acid-base titration method, for lubricating base oil or biodiesel, take 10 mL of sample, add 50 mL of a 1:1 ethanol-ether mixture, add two drops of phenolphthalein indicator, and titrate with 0.1 mol / L potassium hydroxide ethanol solution until the solution turns slightly red and does not fade within 30 seconds. Based on the volume and concentration of the potassium hydroxide solution consumed in the titration, the sample mass, and the molar mass of potassium hydroxide, calculate the acid value = (volume of potassium hydroxide solution consumed in the titration × concentration of potassium hydroxide solution × 56.1) / sample mass. Test results are shown in Table 1.

[0121] Table 1:

[0122]

[0123] 2.1 Composite flocculant acid removal rate test:

[0124] The acid value of waste lubricating oil was determined using an acid-base titration method. 10 mL of each pre- and post-treatment waste lubricating oil was added to 50 mL of a 1:1 ethanol-ether mixture. Two drops of phenolphthalein indicator were added, and the solution was titrated with 0.1 mol / L potassium hydroxide ethanol solution until it turned slightly red and did not fade within 30 seconds. The acid value was calculated as follows: Acid value = volume of potassium hydroxide solution consumed in titration × concentration of potassium hydroxide solution × 56.1 / mass of sample. The acid removal rate was calculated as (initial acid value - acid value after treatment) / initial acid value × 100%. The results are shown in Table 2.

[0125] 2.2 Composite flocculant solid particle removal rate test:

[0126] The solid particle content was determined gravimetrically. 100 mL of waste lubricating oil was vacuum filtered through a 0.45-μm microporous filter membrane, which had been weighed to a constant weight. The filter residue was collected and dried at 105°C to a constant weight. The solid particle content was calculated as (total mass of the filter membrane and filter residue - filter membrane mass) / sample mass × 100%. The particle removal rate was calculated as (initial particle content - post-treatment particle content) / initial particle content × 100%. The results are shown in Table 2.

[0127] Table 2:

[0128]

[0129] Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for recycling waste lubricating oil, characterized in that: The following steps are involved: S1, adding a composite flocculant to waste lubricating oil, heating and stirring, and then performing magnetic separation to obtain pretreated waste lubricating oil; S2, mixing the pretreated waste lubricating oil, glycerol and Ni / Al2O3 catalyst uniformly, then heating under pressure for reaction, and cooling to room temperature to obtain a catalytic hydrogenation product; S3, subjecting the catalytic hydrogenation product to vacuum distillation to recover the lubricating base oil fraction, adding methanol and concentrated sulfuric acid to the distillation residue, heating to carry out esterification reaction, and separating the liquid to obtain biodiesel; The preparation method of the composite flocculant in step S1 comprises the following steps: S11, dissolving ferric chloride and ferric chloride in deionized water, heating to 70-80° C. under nitrogen protection, adding ammonia water, stirring and reacting, and performing magnetic separation and water washing to obtain Fe3O4 nanocrystals; S12, dispersing Fe3O4 nanocrystals in a mixed solution of ethanol and water, adding ethyl orthosilicate and ammonia water, stirring and reacting at room temperature, and performing magnetic separation and drying to obtain Fe3O4@SiO2 composite particles; S13, dispersing Fe3O4@SiO2 composite particles in a hexadecyltrimethylammonium bromide aqueous solution, then adding ethyl orthosilicate, with a molar ratio of ethyl orthosilicate to hexadecyltrimethylammonium bromide of 10:1-3, heating and stirring to react, and then calcining after magnetic separation to remove the template to obtain mesoporous Fe3O4@SiO2 composite particles; S14, adding silane coupling agent KH560 to a mixed solution of ethanol and water, heating and stirring to hydrolyze, then adding mesoporous Fe3O4@SiO2 composite particles, heating and stirring to react, to obtain epoxy-based mesoporous Fe3O4@SiO2 composite particles; S15, dissolving chitosan in acetic acid solution, then adding glycidyl methacrylate, and heating for reaction to obtain double-bond modified chitosan; S16, dissolving the double-bond modified chitosan in an acetic acid solution, then adding the epoxy-group mesoporous Fe3O4@SiO2 composite particles, heating and stirring to react, to obtain grafted double-bond modified chitosan mesoporous Fe3O4@SiO2 composite particles; S17, dispersing the grafted double bond modified chitosan mesoporous Fe3O4@SiO2 composite particles in deionized water, then adding acrylamide and ammonium persulfate, heating and reacting under nitrogen protection to obtain a composite flocculant.

2. The method for recycling waste lubricating oil according to claim 1, wherein: In step S1, the amount of the composite flocculant added is 0.5-3.0% of the mass of the waste lubricating oil.

3. The method for recycling waste lubricating oil according to claim 1, wherein: In the step S2, the mass ratio of the pretreated waste lubricating oil, glycerol and Ni / Al2O3 catalyst is 10:1.5:0.

3.

4. The method for recycling waste lubricating oil according to claim 1, wherein: In step S2, the heating reaction temperature is 220-230° C., and the heating reaction time is 3-4 hours.

5. The method for recycling waste lubricating oil according to claim 1, wherein: In step S3, the reduced pressure distillation temperature is 280° C. and the pressure is 5 kPa.

6. The method for recycling waste lubricating oil according to claim 1, characterized in that: In step S3, the mass ratio of the distillation residue to methanol is 2:

1.

7. The method for recycling waste lubricating oil according to claim 1, wherein: In step S11, the reaction time is 30-50 minutes.

8. The method for recycling waste lubricating oil according to claim 1, characterized in that: In the step S16, the mass ratio of the epoxy-based mesoporous Fe3O4@SiO2 composite particles to the double-bond modified chitosan is 7:2-4.

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